This invention relates to novel compounds useful as sequestrants and scale and corrosion inhibitors, methods of making such compounds, and methods of inhibiting scale formation and corrosion of metals therewith.
The utility of materials having the ability to sequester various ions from aqueous media is well recognized. For example, materials having ability to sequester calcium ions, iron ions, etc., are extensively utilized in treating water to prevent formation of scale or building up of precipitants in boilers, water towers, heat exchangers, etc. Some materials of this type are empirically found to also be useful as corrosion inhibitors. That is, they inhibit the corrosion of metals by water, and especially oxygen-bearing water.
The present invention has special utility in the prevention of the corrosion of metals which are in contact with circulating water, that is, water which is moving through condensers, engine jackets, cooling towers, evaporators or distribution systems, however, it can be used to prevent the corrosion of metal surfaces in other aqueous corrosive media. This invention is especially valuable in inhibiting the corrosion of ferrous metals including iron and steel (also galvanized steel) and nonferrous metals including copper and its alloys, aluminum and its alloys and brass. These metals are generally used in circulating water systems.
The major corrosive ingredients of aqueous cooling systems are primarily dissolved oxygen and inorganic salts, such as the carbonate, bicarbonate, chloride and/or sulfate salts of calcium, magnesium and/or sodium.
Most commercial water contains iron and alkaline earth metal cations, such as calcium, barium, magnesium, etc., and several anions such as hydroxide, bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate fluoride, etc. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products under the conditions of the application (i.e., use), precipitates form until their reaction solubility product concentrations are no longer exceeded. For example, when the concentrations of calcium ion and sulfate ion exceed the solubility of the calcium sulfate, a solid phase of calcium sulfate will form.
Solubility product concentrations are exceeded for various reasons, among which are evaporation of the water phase, change in pH, pressure or temperature, and the introduction of additional ions which form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying system, they form scale. This adherent scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors microorganisms. The presence of this scale is an expensive problem in many industrial water systems (e.g., boilers, cooling towers, evaporators, etc.), oilwells, and the like, causing delays and shutdowns for cleaning and removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Generally, this requires many times as much chelating or sequestering agent as cation, and these amounts under certain conditions are not always desirable or economical.
More than 25 years ago it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts less than the concentrations needed for sequestering or chelating. See, for example, Hatch and Rice, "Industrial Engineering Chemistry", vol. 31, pages 51 and 53; Reitmeier and Buehrer, "Journal of Physical Chemistry", vol. 44, No. 5, pages 535 and 536 (May 1940); Fink and Richardson U.S. Pat. No. 2,358,222; and Hatch U.S. Pat. No. 2,539,305, all of which are incorporated herein by reference. For sequestration, the mole ratio of precipitation inhibitor equivalents to scale forming cation is usually 1:1 or greater (2:1, 3:1, etc.). These ratios are referred to as stoichiometric. Substoichiometric amounts would include all mole ratios of precipitation inhibitor equivalent to scale forming cation that are less than the level required for sequestration; this phenomenon is known in the water treating art as the "threshold" effect.
It is to be understood that the term "threshold" as utilized herein refers to the chemical and/or physical phenomenon that less than stoichiometric quantities of the particular precipitation inhibitor can effectively prevent the precipitation of various metallic ions such as calcium, iron, copper and cobalt and/or alter those crystals formed such that the adherence to surfaces is substantially reduced. In other words, the threshold treatment of water is that technique by means of which less than stoichiometric quantities of the treating agent are added to interfere with the growth of crystal nuclei and thereby prevent the deposition of insoluble deposits.
Consequently, precipitation inhibitors which function as threshold agents as well as sequestering agents and corrosion inhibitors represent an advancement in the art and are in substantial demand.